The troops who died were from the 3rd Marine Aircraft Wing based at Miramar, in southern California.

"The aircraft, an AH-1W 'Cobra' and an UH-1Y 'Huey', were conducting routine training operations" when they collided around 8:00 pm (0300 GMT Thursday), the Marine Corps said.

The crash was under investigation and there were no survivors in the accident, officials said.

The Marines conduct the bulk of their aviation training at the vast Yuma air station in Arizona.

Arizona's senior US Senator John McCain expressed his condolences after the crash.

"My thoughts and prayers are with the families of the seven Marines killed in the helicopter collision last night near Yuma, Arizona," McCain, a former US Navy pilot who was taken prisoner during the Vietnam War, said in a statement.

"This tragedy is a grave reminder of the sacrifices the men and women of our armed forces make to keep us safe -- both in training here at home and in combat abroad."

The crash was the latest mishap for the 3rd Marine Aircraft Wing.

Two Marines were killed in September when their helicopter went down during training at California's Camp Pendleton, north of San Diego, setting off a brush fire.

In August, two Marines had to eject from their F/A-18 fighter jet before it crashed in the Pacific Ocean. They were rescued four hours later.

In July, a veteran Marine aviator was killed when his UH-1Y helicopter crashed at Camp Pendleton.